Many vessels in animals transport fluids from one bodily location to another. Frequently, fluid flows in a unidirectional manner along the length of the vessel. Varying fluid pressures over time, however, can introduce a reverse flow direction in the vessel. In some vessels, such as mammalian veins, natural valves are positioned along the length of the vessel and act as one-way check valves that open to permit the flow of fluid in the desired direction, and quickly close upon a change in pressure, such as a transition from systole to diastole, to prevent fluid flow in a reverse direction, i.e., retrograde flow.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of the valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present, which can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include surgical techniques in which valves are bypassed or replaced with autologous sections of veins with competent valves. These techniques are generally undesirable because they represent a relatively invasive approach to treating valve insufficiency.
Minimally invasive techniques and instruments for placement of intraluminal medical devices have developed over recent years. A wide variety of treatment devices that utilize minimally invasive technology has been developed and includes stents, stent grafts, occlusion devices, infusion catheters and the like. Minimally invasive intravascular devices have especially become popular with the introduction of coronary stents to the U.S. market in the early 1990's. Coronary and peripheral stents have been proven to provide a superior means of maintaining vessel patency, and have become widely accepted in the medical community. Furthermore, the use of stents has been extended to treat aneurisms and to provide occlusion devices, among other uses.
Recently, prosthetic valves have been developed that use a support frame such as a stent. Frequently, a graft member is attached to the support frame and provides a valve function to the device. For example, the graft member can be in the form of a leaflet that is attached to a stent and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction, and in a second direction the valve is closed to prevent fluid flow in a second, opposite direction. An example of this type of prosthetic valve is described in commonly owned U.S. Pat. No. 6,508,833, to Pavcnik for a MULTIPLE-SIDED INTRALUMINAL MEDICAL DEVICE, which is hereby incorporated by reference in its entirety. In other examples of prosthetic valves, a tube that terminates in valve portions is attached to one or more support frames to form a valve. The valve portions open to permit fluid flow in a first direction in response to fluid pressure on one side of the portions, and close to prevent fluid flow in a second, opposite direction in response to fluid pressure on opposite sides of the portions. An example of this configuration is provided in U.S. Pat. No. 6,494,909 to Greenhalgh for AN ENDOVASCULAR VALVE, which is hereby incorporated by reference in its entirety.
One challenge in the development of prosthetic valves is the sizing of the valves. The body vessels in which the valves are placed exist in various sizes and configurations in any given patient population. For example, prosthetic venous valves are designed for placement within a vein of a patient. These body vessels exist in a variety of sizes and have a high degree of elasticity. Accordingly, there is a need for medical devices, kits, and methods that are capable of being adapted to vessels of various sizes.